1. Field of the Invention
The present invention relates to a toner for an electrophotographic image forming apparatus, and more particularly, to a nonmagnetic one-component toner used to develop an electrostatic latent image formed on the surface of a photosensitive medium for an electrophotographic image forming apparatus.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus, such as a copier, a laser printer or a facsimile, forms an electrostatic latent image on a photosensitive medium, such as a photosensitive drum or a photosensitive belt, develops the electrostatic latent image with a developing agent having a predetermined color, and transfers the developed electrostatic latent image onto a sheet of paper, thus obtaining a desired image.
There are two types of electrophotographic image forming apparatuses, such as a dry-type electrophotographic image forming apparatus and a wet-type electrophotographic image forming apparatus, depending on a developing agent. In the dry-type electrophotographic image forming apparatus, a powder state of toner is used as the developing agent, and in the wet-type electrophotographic image forming apparatus, a liquid developing agent in which a toner that is mixed with a liquid carrier is used as the developing agent.
Dry-type developing methods using the powder state of toner include a two-component developing method using a two-component toner in which carrier particles used to transport toner particles are contained, and a one-component developing method using only toner without a carrier. The one-component developing method includes a magnetic one-component developing method and a nonmagnetic one-component developing method. In the magnetic one-component developing method, a developing operation is performed using a toner for the magnetic one-component development. In the nonmagnetic one-component developing method, a toner layer is formed on a developing roller using a toner for the nonmagnetic one-component development and is developed either in contact with not in contact with a photosensitive medium.
In the contact-type nonmagnetic one-component developing method, the price is very competitive. However, since it is difficult to attain dot reproducibility, line reproducibility, and high-resolution implementation, it is not easy to obtain a high quality image. Meanwhile, in the noncontact-type nonmagnetic one-component developing method, the structure of a developing unit is simple, and thus may be minimized. In addition, since attaining color reproducibility, edge reproducibility, high tone gradation, and high-resolution implementation is facilitated, a high quality image may be obtained.
FIG. 1 schematically illustrates a noncontact-type developing unit for a conventional electrophotographic image forming apparatus. Referring to FIG. 1, the conventional electrophotographic image forming apparatus includes a photosensitive medium 10, a charging roller 12, a laser scanning unit (LSU) 14, a developing roller 16, a toner supplying roller 18, and a toner layer regulation unit 20.
The photosensitive medium 10 has a structure in which a photosensitive film formed of a photosensitive material is formed on the circumference of a metallic drum. The surface of the photosensitive medium 10 is charged by the charging roller 12 to a predetermined potential, and an electrostatic latent image is formed on the surface of the charged photosensitive medium 10 by light irradiated from the LSU 14.
Toner 30 stored in a toner storage space 32 is supplied by the toner supplying roller 18 to the surface of the developing roller 16. The toner 30 supplied to the surface of the developing roller 16 becomes a thin film having a uniform thickness using the toner layer regulation unit 20. Simultaneously, the toner 30 is rubbed by the developing roller 16 and the toner layer regulation unit 20 and is charged with a predetermined charge. In this case, M/A (mg/cm2) and Q/M (μC/g) of the toner 30 are regulated by the toner layer regulation unit 20. Here, M/A (mg/cm2) is the weight of the toner 30 per unit area measured on the developing roller 16 after going through the toner layer regulation unit 20, and Q/M (μC/g) is an amount of charge of the toner 30 per unit weight measured on the developing roller 16 after going through the toner layer regulation unit 20.
As described above, the toner 30, which is charged with a predetermined charge and in which M/A (mg/cm2) and Q/M (μC/g) are regulated, moves to the surface of the photosensitive medium 10 using the developing roller 16 that is spaced a predetermined gap apart from the photosensitive medium 10 and rotated. In this case, the movement of the toner 30 is performed by a potential difference between the developing roller 16 and the electrostatic latent image formed on the surface of the photosensitive medium 10. The toner 30 that moves to the surface of the photosensitive medium 10 is attached to the electrostatic latent image. As such, the electrostatic latent image is developed as a desired image.
The image developed on the surface of the photosensitive medium 10 is transferred onto a sheet of paper by a transfer roller (not shown), and then is fused on the sheet of paper by a fusing unit (not shown). Toner remaining on the surface of the photosensitive medium 10 after the image is transferred onto the sheet of paper is removed by a cleaning blade 22 and is stored in a waste toner storage space 34.
Nonmagnetic one-component polymerization and pulverization-type toner used in the above-described conventional noncontact-type developing method includes toner particles in which a coloring agent, a charging control agent (CCA), and a release agent are added uniformly into a binder resin to improve chromaticity, charging characteristics, and fusing properties, and a variety of types of external additives added to toner particles to provide the fluidity, the charging stability, and the cleaning properties of toner.
In the noncontact-type nonmagnetic one-component developing method, to maintain stable developing properties, prevent contamination (fog or background) on a nonimage portion, and prevent the scattering of toner, the charging amount of toner should be maintained uniformly, and the distribution of the charging amount of the toner should be maintained uniformly both at an initial developing stage and after a long-term image printing operation. In this way, to provide uniform charging properties to toner, toner should be formed to a small thickness on a developing roller. However, when a toner layer is formed to a thin film on the developing roller, the toner may easily deteriorate due to a large amount of stress, or may be easily fused on a toner regulation unit. In addition, when the toner layer is formed to a small thickness on the developing roller, a developing efficiency may be rapidly lowered due to an increase in a toner charging amount, and the concentration of an image may be thereby lowered. When the toner charging amount is reduced to improve the developing efficiency, an increase in contamination (fog) on the nonimage portion and contamination caused by the scattering of toner occur.
Accordingly, in the noncontact-type nonmagnetic one-component developing method, the charging amount of the toner should be maintained uniformly, and the distribution of the charging amount of the toner should be maintained uniformly so that the occurrence of contamination (fog) on the nonimage portion is prevented, and excellent developing properties are maintained even after the long-term image printing operation. This has a close relation to the type and composition of an external additive added to toner particles.
For example, Japanese Patent Laid-Open Publication No. 2000-122336 discloses a two-component negative charge type toner in which two or more external additives which are at least positive charge type inorganic particles of 80-800 nm number average diameter and negative charge type inorganic particles of 5-50 nm number average diameter, where the weight ratio of the positive charge type inorganic particles to the negative charge type inorganic particles is in a range of 2.5:7.5 to 7.5:2.5, are mixed with a carrier. In addition, Korean Patent Laid-Open Publication No. 2002-061682 discloses a nonmagnetic one-component toner composition in which an external additive including hydrophobic silica having a specific surface area of 20-80 m2/g, hydrophobic silica having a specific surface area of 130-230 m2/g, and titanium oxide having an average diameter of 100-500 nm is added to the surface of toner particles.
In the related art, to grant fluidity to the toner, prevent an increase in a toner charging amount, and remove a low electrical resistance material, such as remaining toner, fat or ozone adducts attached to a photosensitive medium and a toner layer regulation unit, as described above, silica particles and two or three types of inorganic fine particles are used as external additives. The inorganic fine particles are effective as an abrasive to provide a cleaning effect, an initial toner charging property, and fluidity. However, after long-term use, the improvement in charging stability and in the transfer property of the toner are not sufficient. In addition, even though the size of the inorganic particles is very small, the inorganic particles easily cohere to one another. Thus, it is easy to form a cohesive substance having the size of the coarse particles of several tens of μm, and it is difficult to attach the cohesive substance onto the surface of the toner particles electrostatically. Thus, a larger energy is needed to attach the inorganic particles that are joined together onto the surface of the toner particles. In this case, the inorganic particles are easily buried under the surface of the toner particles. Meanwhile, when the inorganic particles do not stick sufficiently to the surface of the toner particles, the inorganic particles separate from the toner particles and are accumulated in the toner stored in a developing unit, and due to white coarse particles formed of inorganic fine particles that are joined together, a white point appears on a printed image after a developing or fusing operation. In particular, when only hydrophobic titanium dioxide TiO2 ultrafine particles are added as an external additive together with silica, image defects, such as offset and line burst, occur, and the characteristic of prevention of contamination (fog) on the nonimage portion is lowered after a long-term operation.
Meanwhile, positive charge type polymer beads may be added so that the charging stability of the toner and the uniform distribution of a toner charging amount are maintained for a long time. However, since the polymer beads have the size of coarse particles, unlike silica particles having a size equal to or less than 50 nm, the polymer beads easily separate from the toner particles. In particular, the polymer beads attached to the photosensitive medium are not easily cleaned due to a spherical shape. As such, the polymer beads remaining on the photosensitive medium are attached and accumulated to a charging roller, causing contamination of the charging roller and image contamination.